More methods to work with uncorresponding bits

[rusty-snake]

• complement with from_bits_retain

  FlagsT::from_bits_retain(!FlagsT::all().bits())

• contains_uncorresponding_bits or has_uncorresponding_bits

[KodrAus]

Hi @rusty-snake 👋 Are you able to share any more details on what you're trying to do here?

[rusty-snake]

Yes, my use case are flags of ffi functions where I can only allow known safe flags. Passing known unsafe or flags added by future versions might be unsafe. Instead of silent truncating the bits I want to check and return an error.

[KodrAus]

Ahh, so you're suggesting a function to detect whether or not there are any bits set that don't correspond to a flag? Would it be enough to write?

let bits = Flags::from_bits(possibly_invalid.bits()).ok_or_else(|| "contains unrecognised bits")?;

[rusty-snake]

Yes, that would be an alternative way to implement it.

[KodrAus]

Hi @rusty-snake. Just calling out that 2.3.3 slightly alters the behavior of the ! operator that affects flags with bits that only partially correspond to a known flag. I'm not sure if that will affect you, but wanted to draw your attention to it.

In #363 (comment) we're trying to come up with a fully consistent treatment of unknown bits, which might become a 3.0.0 release. That would let us include some new inherent methods for detecting and truncating them if you wish.

[KodrAus]

In #400 we added strict methods to the parser and formatter to work with flags that only want bit patterns that fully contain some number of defined flags. We could consider adding from_bits_strict as a variant that's even tighter than from_bits and from_bits_truncate. The difference only matters for multi-bit flags, but is outlined here.

I've also found myself wanting a .truncate() inherent method on flags types too.